This invention relates generally to solid state microwave devices and in particular to a TUNNETT diode and the method of making it.
The use of solid state electronic devices for microwave application has been the object of research and development effort for many years. Solid state devices are generally considered to be less expensive, more compact, more rugged, more reliable and more efficient than other active microwave devices such as travelling wave tubes. Although the frequency response of normal semiconductor devices such as transistors has been increased in recent years to the low gigahertz range, many microwave systems are operating at frequencies up to 40 GHz. Furthermore, a crowded electromagnetic spectrum and high data rates are forcing the consideration of use of millimeter and submillimeter microwaves. The millimeter band extends from 30 GHz to 300 GHz while the submillimeter band extends to over 1000 GHz. If sufficiently good microwave devices can be made for these frequencies, their use will provide additional benefits such as compact equipment, small antennas, high directionality and low power requirements.
Several special microwave devices have been discovered which operate at frequencies in the submillimeter as well as millimeter range. Many of these are discussed in S. M. Sze's second edition of Physicsof Semiconductor Devices, Wiley, 1981. One of these types of special microwave devices is the tunnel diode which is heavily doped on both sides of its p-n junction. This junction is made abrupt so that there is a large probability of quantum mechanical tunneling across the p-n junction. As long a majority carrier electron states in the conduction band on the n-side (or hole states in the valence band on the p-side) are present at energies for which there are electron (or holes) states on the other side of the junction, current increases with increasing voltage. However, if the forward voltage is raised to the point that the bands on one side have energies corresponding to the band gap on the other side of the tunneling junction, there are no states to which to tunnel. As a result, the current-voltage (I-V) curve reaches a peak and then falls. In the region beyond the peak, the static resistance, I/V, falls. Thus if the tunnel diode is biased at an operating point on the falling I-V curve, and has small oscillations about the operating joint, the negative dynamic resistance, dI/dV, will cause the tunnel diode to act as an amplifier.
Another general class of special microwave device relies on transit time delays. There are several specialized types of transit time devices such as an IMPATT (impact ionization avalanche transit time), BARITT (barrier injection transit time) and TUNNETT (tunneling transit time). The last mentioned device is the subject of this application. All of these devices rely to greater or lesser extent on the finite transit time that an injected pulse of carriers takes to cross a drift region. When operated at a sufficiently high frequency, the injected current pulse is collected out of phase from the impressed voltage pulse. The non-linear characteristics of such a device can be used for applications such as mixers and detectors. With proper design these devices can be shown to exhibit negative dynamic resistance for some range of frequencies. Thus they can operate as microwave amplifiers.
A unified analytic model of IMPATTs, BARRITs and TUNNETTs has been provided by P. J. McCleer et al. in Solid State Electronics, volume 24, pages 37-48, 1981 and by M. E. Elta and G. I. Haddad in IEEE Transactions on Electron Devices, volume ED-26, pages 941-947, 1979.
Such devices operate in the TUNNETT mode when their parameters are such that pure tunneling is present and the avalanche multiplication is relatively less important than tunnelling. A TUNNETT is particularly useful as a low-noise amplifier, a medium power oscillator, a self-oscillating mixer and a detector, particularly at millimeter frequencies.
Previously produced TUNNETTs have utilized a HI-LO-HI structure, i.e. the injector region and collector region have a relatively high doping concentration of the same conductivity type. The drift region between the injector and collector region is either intrinsic or of a relatively low doping concentration. Because these devices were fabricated by vapor phase epitaxy (VPE) the width of the injector region was no less than 100 nm and had a graded doping profile. As a result, performance was limited by the lack of a hyper-abrupt interface between the injector and drift regions and the inability of VPE to grow thin highly doped injector regions. The frequency response is limited by the transit time, .tau..sub.D =d/v.sub.s where d is the combined width of the injector and drift regions and v.sub.s is the saturated carrier velocity, approximately 10.sup.7 cm/s. TUNNETTs made previously by VPE, liquid phase epitaxy, and molecular beam epitaxy had high junction capacitance and series resistance which lead to rapid deterioration and burnout when these devices were made to operate at high frequencies. Furthermore previous methods of fabricating TUNNETTs required that the device area scale down with frequency. As a result, a requirement for constant power or current at higher frequencies meant that the power density through the TUNNETT was increased causing increased susceptibility to burnout.